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(American Journal of Pathology. 2007;170:293-300.)
© 2007 American Society for Investigative Pathology
DOI: 10.2353/ajpath.2007.060530

Unique Matrix Structure in the Rough Endoplasmic Reticulum Cisternae of Pseudoachondroplasia Chondrocytes

Thomas M. Merritt*, Roger Bick{dagger}, Brian J. Poindexter{dagger}, Joseph L. Alcorn* and Jacqueline T. Hecht*{ddagger}

From the Departments of Pediatrics* and Pathology,{dagger} University of Texas Medical School at Houston, Houston; and Shriners Hospital for Children,{ddagger} Houston, Texas

Mutations in cartilage oligomeric matrix protein (COMP) cause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED/EDM1). Because COMP exists as a homopentamer, only one mutant COMP subunit may result in an abnormal complex that is accumulated in expanded rough endoplasmic reticulum (rER) cisternae, a hallmark of PSACH. Type IX collagen and matrilin-3 (MATN3), also accumulate in the rER cisternae of PSACH chondrocytes, but it is unknown how mutant COMP interacts with these proteins. The studies herein focus on defining the organization of these intracellularly retained proteins using fluorescence deconvolution microscopy. A unique matrix organization was identified in which type II procollagen formed a central core surrounded by a protein network of mutant COMP, type IX collagen, and MATN3. This pattern of matrix organization was found in multiple cisternae from single chondrocytes and in chondrocytes with different COMP mutations, indicating a common pattern of interaction. This suggests that stalling of mutant COMP and an interaction between mutant COMP and type II procollagen are initiating events in the assembly of matrix in the rER, possibly explaining why the material is not readily cleared from the rER. Altogether, these data suggest that mutant COMP initiates and perhaps catalyzes premature intracellular matrix assembly.




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K. L. Posey, K. Hankenson, A. C. Veerisetty, P. Bornstein, J. Lawler, and J. T. Hecht
Skeletal Abnormalities in Mice Lacking Extracellular Matrix Proteins, Thrombospondin-1, Thrombospondin-3, Thrombospondin-5, and Type IX Collagen
Am. J. Pathol., June 1, 2008; 172(6): 1664 - 1674.
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Copyright © 2007 by the American Society for Investigative Pathology.